A constructional element of this kind is known from German Offenlegungsschrift No. 32 33 654.
More stringent conditions for the protection of the environment require traffic noise to be minimized as far as possible, and in particular sound emission from motor vehicles to be reduced by measures in the region of the main source of noise, namely the engine. Various proposals have recently been made to this end.
The desired reduction in external noise in the engine compartments of motor vehicles is undertaken in particular with the aid of bulkheads beneath the engine that close off what would otherwise be free surfaces for the escape of sound by an acoustically effective element. While a procedure of this kind certainly generally decreases the external sound level, there is a risk for the user that as a result of the shape of such a bulkhead beneath the engine, which neither is nor can be optimized, the noise level inside the passenger compartment will be increased. It is generally known that this negative effect can be prevented by providing the bulkhead beneath the engine, as well as the freely accessible walls of the engine compartment, with a material or system that absorbs airborne sound by means of which the sound reflections that otherwise occur with smooth boundary surfaces are avoided and the overall sound level is lowered.
Previous embodiments of such acoustically effective engine bulkheads have as a rule employed supporting shells having high mechanical strength, which require relatively expensive materials and costly manufacturing processes. Such shells are preferably provided with foam-based absorbers for airborne sound, e.g. in the form of directly applied compressed foam having laminated surfaces, or with compressed and molded foam foils spaced away from the carrier shell. Known possibilities for tuning absorbers of foam- or fibre-base materials are available to the man skilled in the art from the literature. The factors that can be influenced can be found in any appropriate textbook.
The prior art mentioned above (cf. German Offenlegungsschrift No. 32 33 654) provides sound-absorbing constructional elements with flat surfaces and an internal cellular structure, consisting essentially of foils of strictly geometrical form, as a result of which the surfaces of these foils can be excited to loss-making vibrations (or oscillations). These loss-making vibrations' which are used as a means of absorbing airborne sound, are bending vibrations of the so-called plates or boundary surfaces, as is explained in German Pat. No. 27 58 041 (column 3, lines 33-45).
The basic idea that sound absorption is effected by natural plate vibrations of the surfaces--mainly the bottom surfaces--of the strictly geometrical chambers formed by foils, is explained in detail (German Offenlegungsschrift No. 32 33 654, page 4, lines 11-17). The frequency band width of such absorbers, however, is obviously very narrow unless additional steps are taken. It is also disclosed (loc. cit., page 7, lines 34/35 and page 8, lines 1-6) how such an absorber can be tuned to given spectra, e.g. of motor vehicles, which generally need a wide-band treatment. The chambers should be divided by pits or depressions, preferably in a cruciform arrangement, into a plurality of sub-chambers, which then together lead to broadening of the absorption band without its absolute value being lowered.
Constructional elements of the kind described above suffer from various disadvantages. For example, the use in the usual way of expensive high-strength plastics together with equally costly production processes, due for example to the incorporation of glass fibre in the materials, is obviously already a disadvantage. This disadvantage is particularly serious in mass production. It also has to be taken into account, in the case of foam-based absorbers, that if the surface is damaged the underlying foam can quickly become saturated with moisture, for example water, but also gasoline or oil. The efficiency of absorption is thereby greatly and noticeably reduced, and in addition saturation with inflammable substances is a source of considerable danger that has to be avoided. While the flat constructional elements described are acoustically effective and can be tuned to predetermined frequency bands, the external geometry of the bodywork that is laid down as essential by the motor vehicle manufacturer prevents such flat elements being used as fully functional vehicle parts with a required acoustic efficiency. The use of the cellular structures described requires flat elements with strictly geometrical grids. There is the additional condition that the boundary surfaces of the chambers formed by the cellular structure must still be capable of vibration, i.e. of being excited to bending vibrations. In fact it is mentioned in German Offenlegungsschrift No. 30 30 238 (page 8, last paragraph) that a base closure plate can be made as a very stiff, rigid plate. This is explained using as an example of flat coating surfaces the walls of tunnels, etc. that have previously been prepared using suitable supporting walls. In this case however, at least some of the faces of the chamber must be capable of being excited to natural plate vibration.